For a number of years it has been known that various microorganisms produce metabolites exhibiting biological activity so as to be useful as biocides for combating diseases and pests.
Especially it has been known that some species of the genus Curvularia produce different metabolites exhibiting biological activity, in particular as phytotoxins. However, none of these metabolites are known to be fungicides. C lunata (Wakk.) Boed was found to produce at least two phytotoxins. They cause necrotic spots on plant leaves even when they are highly diluted. (F. Macri and A. Vianello, Physiological Plant Pathology 8:325-331, 1976). The toxins are phytotoxins and are non host specific. Host specific toxins have been isolated from culture filtrates of C. pallescens Boedijn and from diseased host leaves attacked by this species (Olufolaji, Cryptogamie Mycol. 7(4):335-342, 1986). These toxic substances gave chlorotic spots to maize plants. They later became necrotic similar to in-vivo symptoms of the disease caused by C. pallescens.
Curvularin, a macrocyclic metabolite, has been isolated from a Curvularia species (O. C. Musgrave, J. Chem Soc., 1956, 4301) and from C. lunata (Coombe et al, Aust. J. Chem. 21:783, 1968). The compound has the formula C.sub.16 H.sub.20 O.sub.5 (molecular weight 292.3) and has the formula 1 below: ##STR2## Crude curvularin was isolated as colourless or pale yellow-brown crystals.
Neither curvularin nor the crude filtrates from Curvularia showed anti-bacterial activity (against Bacillus subtilis, Escerichia coli, and Staphylococcus aureus) or anti-fungal activity (spore germination test with Penicillium digitatum).
None of the above compounds are useful for combating plant diseases and pests because they are strong phytotoxins.
Although progress has been made in the field of identifying and developing biological biocides for combating various diseases and pests in plants and animals of agronomical importance, most of the biocides in use are still synthetic compounds that are difficult to decompose in nature and have a broad spectrum of activity.
In the last decade or so there has been a still increasing concern for the impact of such biocides on the environment and the ecosystems surrounding farmlands, and consequently there exists an outspoken need for biocides that are more specific in their activity, and are readily degradable in the natural environment.
A number of compounds related to the compounds disclosed through this invention have been disclosed in European Publication No. EP 360 760 (Sandoz AG), published 28 Mar. 1990. These compounds are disclosed as being produced by the fungal genus Septoria, and as having interesting pharmacological properties, e.g. antifungal activity in animals against yeast and yeast-like species causing systemic attacks. The publication contains only reference to one test against Candida, and no reference to which of the 82 exemplified compounds were tested for this activity, but states that the most preferred compound can be represented by the formula: cyclo-[Pec-MeVal-Val-MeAsp-MeIle-MeIle-Gly-MeVal-TyrOMe-D-Lact] (Example 1, first compound).